1. Field of the Invention
The present invention relates to an optical disk from/onto which data is reproduced/recorded by utilizing laser light; an optical disk apparatus which reads out or records signals using an optical disk; and a method for producing an optical disk master.
2. Description of the Related Art
In recent years, an optical disk has been widely used as a medium for storing mass data files such as music, an image, and the like. Studies for increasing the capacity of an optical disk have been made in order to realize a wider range for its use.
At present, due to the partial response technique, a recording density of an optical disk apparatus is about 0.4 μm/bit in a linear density direction, and about 1.2 μm/track.
In order to realize a higher density optical disk, various methods have been suggested. One of such methods is a super-resolution readout method in which a reproducing film of an optical disk has a super-resolution reproducing effect (see U.S. Pat. No. 5,168,482). The “super-resolution reproducing effect” refers to high-resolution reproduction wherein its resolution level exceeds the resolution level for collected light beam. Such high-resolution reproduction is realized by using a reproducing film heated by a light beam collected onto an optical disk medium, which has a function of generating a reproduction signal in a region having a predetermined temperature. According to the super-resolution readout method, it is possible to reproduce a signal at a resolution level higher than the resolution of a light beam, which is determined by the wavelength of its light source and the aperture of its collective lens. Thus, it is possible to realize a high-density optical disk apparatus. Specifically, according to the super-resolution readout method, a recording density can be improved up to about 0.2 μm/bit in a liner density direction and about 0.6 μm/track.
In order to produce an optical disk according to the super-resolution readout method, it is necessary to preformat addresses (e.g., sector numbers or track numbers which are recorded in sector identification regions) by providing emboss pits on the disk. However, such emboss pits do not provide the super-resolution effect. As a result, when an address is recorded in a narrow track pitch of about 0.6 μm, errors are generated due to cross-talk from an adjacent track. Consequently, the address cannot be reproduced. In order to avoid such a problem, an optical disk in which addresses are recorded so as not to be adjacent to each other between two adjacent tracks has been suggested (e.g., U.S. Pat. No. 5,422,874).
Such optical disks wherein addresses are previously recorded by emboss pits are made by producing a master first and making reproduction using the master by means of injection or the like. Such a master is generally produced as follows. A photoresist film is formed on a circular glass substrate. A light beam such as an argon laser, krypton laser, or the like is collected onto the photoresist film while the glass substrate is being rotated in a precise manner. Portions of the photoresist film such as the grooves or the pits, which are exposed to light by the collected light, are photosensitized. Thereafter, the photoresist film is developed, thereby obtaining a photoresist film having a predetermined pattern (i.e., grooves or pits). Using the thus-obtained photoresist film as a mask, the glass substrate is etched so as to form grooves, emboss pits, and the like on the surface of the glass substrate. Next, the resist film is removed, and the surface of the glass substrate is then made to be conductive. Finally, electrodeposition of nickel or the like is performed for the conductive glass substrate. In the manner as described above, the optical disk master is produced.
In recent years, upon producing a high density master, an electron beam is often used instead of a light beam.
According to the optical disk disclosed in U.S. Pat. No. 5,422,874, address regions are positioned along respectively different radial directions between adjacent two tracks in order to realize a narrow track pitch. Therefore, the address region is required to have a length twice as long as the actually used data length of the address region at the expense of the capacity of the optical disk.
According to an optical disk wherein a data recording region is formed of a groove or a land (e.g., the optical disk disclosed in U.S. Pat. No. 5,422,874), it is extremely difficult to produce a master thereof. Specifically, in order to improve reproduction characteristics of emboss pits of address regions, each of the emboss pits needs to have a width of about 0.3 to about 0.4 μm. On the other hand, a groove region serving as data recording region needs to have a width of about 0.6 to about 0.7 μm which is about the same as the track pitch in order to equalize the reproduction characteristics at the groove region with that at the land region. According to the conventional method for producing an optical disk master, however, since the master is generally produced using one light beam or one electron beam, the width of the region exposed to light, which corresponds to the line width of such a beam, is constant. Therefore, it is extremely difficult to change the width of an address region from the width of a groove region.
In order to make the width of the address region and the width of the groove region different from each other, a method using two light beams having respectively different spot diameters has been known. However, in the case where a master is produced using two light beams, it is necessary to perform the alignment of the two light beams having different widths at an accuracy of about 0.1 μm or less. Thus, the production of the master is extremely difficult. Moreover, the width of the groove region on the master can only be made, at most, about 1.5 times as large as that of the address region.